In a flue gas purification system, the solid, liquid or gaseous pollutants are separated from the flue gas with the aim of reducing damage to the environment to an acceptable degree. In principle, the separation of polluting particulates, from flue gas is effected by mechanical and electrical processes, as well as by the wet dust removal processes such as absorption, adsorption and chemosorption. Basic methods for gaseous pollutant removal from flue gas are absorption, adsorption, condensation and chemical reaction processes. The wet scrubbing processes produce sludges, dissolved solids and waste water.
Frequently flue gas purification systems combine several basic principles into one overall process. One of the most popular refuse burning processes for treating, for example, municipal waste, provides for the removal of particulates from flue gas by means of electrostatic filters or fabric filters and the partial removal of the gaseous pollutants HCl, SO.sub.2, and HF from the flue gas by means of acid or alkali treatment, either upstream or downstream from the filter system (Abfallwirtschaft an der TU Berlin [Waste Treatments at the Technical University of Berlin], Volume 7, pages 1-41). Heavy metals present in gaseous form, e.g. Hg, and organic contaminants, e.g. chlorinated dioxins, have so far only been reliably removed by wet processes downstream from the filters.
The mass-balance for the most important heavy metals in municipal refuse is illustrated in FIG. 3. The composition of the flue gas before purification is given in Tab. 1. An extreme drawback of the conventional methods is the fact that about 30 kg of smoke particulates having high halogen contents are produced for each ton of refuse burned. These particulates also contain heavy metal substances, sometimes in easily transported form. For this reason, the use of particulates from flue gas solids removal systems is decreasing steadily, and it is believed that such materials should only be stored in special depositories under strict safety regulations. The safe removal of the heavy metal contaminants from the ecosystem is nevertheless not assured, particularly with a view toward long-term storage. A primary problem with respect to the contaminants Hg, Cd, Pb, Sb, Sn, Zn, and others in flue ash, in flue gas, and in refuse from combustion systems continues to exist.